Dimming of fluorescent lamps has been achieved by modifications to electronic ballast designs for both mood effect and for energy conservation. Little or no change was required to the standard pure mercury, 32-watt, 4 foot T8 fluorescent lamp. Not all fluorescent lamps can be dimmed in a similar fashion. One problem is that the mercury vapor pressure is difficult to control under dimming conditions, since temperatures in the lamp envelope are significantly lowered.
In fluorescent lamps, optimum performance is dependent on controlling the mercury vapor pressure. The light output reaches a maximum at a specific mercury vapor pressure. The mercury vapor pressure increases with the temperature of the coldest spot inside the lamp envelope (the cold spot). The optimal cold spot temperature in the case of pure mercury is typically in a range of 38° to 42° C. To optimize light output, it is desirable to control the cold spot temperature in this range. Light output is reduced for cold spot temperatures above or below the optimum value.
Many compact fluorescent and high-output lamps have higher temperatures within the envelope due to relatively high power per unit volume. This requires special adaptations or the use of amalgams to achieve optimum mercury vapor pressure and performance. The optimum cold spot temperature for an amalgam is typically about 90° C.
In electrodeless fluorescent lamps, optimum performance is dependent on controlling mercury vapor pressure as in linear fluorescent lamps. Thus far, with the exception of very low power electrodeless lamps, amalgams have been selected to maintain optimum mercury vapor pressure.
The dimming of electrodeless fluorescent lamps by pulse width modulation utilizing amalgams incurs the problem of significantly reduced amalgam temperatures. The desire to operate at low temperature, such as −20° C., and with dimming to as low as 25% of the light output of the undimmed lamp may have the additional effect of producing a secondary cold spot which can deplete the amalgam of mercury and yield control of mercury vapor pressure to the secondary cold spot. Use of pure mercury rather than an amalgam eliminates the secondary cold spot under such conditions but reduces performance at +25° C., 100% duty cycle due to high mercury vapor pressure.
In the production of a sealed lamp envelope, an exhaust tube is used to evacuate and backfill with the desired gas. In other cases, particularly for pure mercury lamps, a tube is added to the lamp envelope to create a cold spot. The tube can be located far enough from the plasma so that temperature is appropriate for location of an amalgam or in some cases pure mercury. In some cases, the location and length of the exhaust tube can be adjusted to achieve sufficient distance from heat sources such as the plasma, driver and electrical circuits. In other cases, the manufacturing process, handling damage concerns and/or aesthetics preclude certain locations or lengths of the exhaust tube. Operating temperature range and dimming must also be considered in order to meet desired mercury vapor pressure to achieve performance requirements.
U.S. Pat. No. 6,172,452, issued Jan. 9, 2001 to Itaya et al., discloses a low pressure mercury vapor discharge lamp wherein an amalgam container and the base are connected by a heat conductive component to control amalgam temperature. U.S. Pat. No. 6,433,478, issued Aug. 13, 2002 to Chandler et al., discloses an electrodeless fluorescent lamp wherein the mercury pressure is controlled in the lamp envelope by the temperature of the amalgam positioned in a tubulation or by the temperature of pure mercury located in the cold spot. U.S. Pat. No. 6,359,376, issued Mar. 19, 2002 to Hollstein et al., discloses a fluorescent lamp wherein a thermally conducting material in the form of a coating of foil on the discharge tube in the region of one or both electrodes is used to achieve optimum operation. U.S. Pat. No. 5,808,418, issued Sep. 15, 1998 to Pitman et al., discloses a control mechanism for regulating the temperature of a fluorescent lamp tube. The control mechanism includes a cold spot mechanism defining a cold spot, a heating mechanism, a power supply and a temperature sensor. U.S. Pat. No. 5,773,926, issued Jun. 30, 1998 to Maya et al., discloses an electrodeless fluorescent lamp wherein the cold spot is maintained at a desired temperature by utilizing a portion of the induction coil to heat the amalgam. U.S. Pat. No. 5,581,157, issued Dec. 3, 1996 to Vrionis, discloses a lamp envelope for an electrodeless discharge lamp having a protuberance such that the cold spot of the lamp envelope is located in the protuberance.
All of the known prior art techniques for controlling cold spot temperature have had one or more drawbacks, including but not limited to limited operating ranges, excessive complexity and difficulties in production. Accordingly, there is a need for improved cold spot structures and control methods for electrodeless fluorescent lamps.